The main absorbent part of most disposable sanitary products or disposable diapers is the pad, or core. The pad is often made of wood pulp that has been fiberized by a special mill, designed to handle fluffing pulp. In the past, this pad was usually made of wood pulp. Now, it is more common that the pad is made of some combination of wood pulp and absorbent polymer. After the pulp is fiberized, the resulting pulp fluff is drawn out of the mill onto a forming screen. The pad is formed on the screen in a forming chamber, in which the pulp fluff and polymer is placed on the forming screen and is forced into a compact configuration by suction of air through the screen. After the pad is formed on the screen, it moves through a set of profiling rolls and on to the folding and packaging part of the converting machine.
The air that is pulled through the forming screen from the pulp mill contains small amounts of fiberized pulp fluff and, in many cases, absorbent polymer. Experience has shown that the amount of the waste fluff and/or polymer that comes through the forming screen is 2% or 3% of the total amount of fluff and/or polymer that enters the forming chamber.
Several filters have been developed for filtering the waste particulates out of the air exiting the forming chamber. These filters have several advantages. First, the filters clean the air that comes from the forming screen and return the air to the plant area or vent the air outside of the plant.
Second, the waste particulates that come through the forming screen are recovered and returned to the mill or forming chamber. The recovered particulates that are returned to the process represent a substantial cost savings to the manufacturer.
For uniform pad formation to take place in the forming chamber, the volume of air moving through the forming chamber and the pressure of that air should be consistent. If the air volume or pressure is changed, the pad will have different thicknesses and absorbencies and may not meet specification. By assisting in moving air through the forming screen, a properly-constructed filter can help to assure air volume and air pressure consistency through the forming chamber.
One of the filters that is used for removing the waste pulp particulates from the air that moves through the forming screen is the rotary drum variety, such as is depicted in FIG. 1. In many cases, when the air contains large quantities of waste particulates, conventional rotary drum filters, when used as a first stage filter, quickly become loaded and undergo severe drops in efficiency. In these cases, the air may first pass through a pre-separator, such as a cyclone, condenser, etc., to remove larger and/or heavier particulates prior to the rotary drum filter stage.
As can be seen in FIG. 1, the process air from the forming chamber of the production machine with the waste particulates entrained are fed through a conduit 12 into a drum filter enclosure 14. The conduit 12 may feed the air and entrained waste particulates to the filter enclosure 14 from the top as shown or may deposit the air and entrained waste particulates from the forming chamber at an opening along the bottom of the filter enclosure 14. A rotary drum 16, which includes a filtration media 18 along its outside, rotates within the drum filter enclosure 14. One end of the rotary drum 16 is closed off (not shown in FIG. 1). The other end of the drum opens to a compartment (not shown in FIG. 1) which houses one or more clean air, or main, fans for withdrawing the air from the filter enclosure 14. The main fan (not shown in FIG. 1) is used to pull air through the filtration media 18 and then through the open end of the drum 16.
As the drum 16 rotates and as the clean air is pulled through the medium 18, particulates 19 settle on the filtration media 18. These particulates 19 are vacuumed off the filtration media 18 through a suction nozzle 20 by a purge fan 22. This fan 22 and another conduit 24 then route the particulates 19 back to the production line and/or to an offline collection system for disposal. The clean air, which is pulled through the filtration media 18 by the fans of the system, is returned to the plant area or is exhausted outside the plant.
The clean air fan, or main fan, is used at the open end of the rotary drum filter for pulling the particulates onto forming screen, pulling the waste particulates to the filter enclosure, and pulling the waste particulates onto the filtration media 18 of the drum filter. In addition, a material handling fan may be used to move the forming air and particulates from the mill through the forming chamber. The material handling fan, also known as a forming fan, is located on the conduit 12 extending from the forming chamber to the drum filter.
The drum filter enclosure generally can only handle approximately 12 inches water column (wc) of negative pressure. The material handling fan must be used if the forming chamber requires more than 8 inches we of negative pressure. If a material handling fan is used in the forming chamber, then the fan at the rear end of the drum filter, or the clean air fan, is used as a balancing fan to keep the filter under a negative pressure. Because increasing forming chamber pressure is a common requirement of sanitary products machine manufacturers, material handling fans are often used to generate the required high pressures and volumes in a system. In such systems, the clean air fan located at the end of the rotary drum filter serves mainly as a balancing fan to keep the filter under negative pressure.
One of the problems found in the rotary drum filter systems is that waste particulates 19 have a tendency to accumulate in bottom corners 26 of the filter enclosure 14. Because of gravity, the waste particulates 19 have a tendency to remain in these corner areas. Further particulates 19 stick to the accumulated particulates, and the problem is compounded. Manufacturers are often forced to shut down the line and clean out this particulate accumulation.
One manner of avoiding particulate accumulation in the corners 26 is by providing a baffle 28 in the corner of the enclosure to decrease the area in which particulates 19 can accumulate. Another method of preventing some of the particulate accumulation utilizes placement of the conduit 12 at the bottom of the enclosure 14. In this manner, a turbulent blast of air is created at the bottom of the filter enclosure, which somewhat prevents the accumulation of particulates 19 on the floor in places in direct contact with the turbulent air stream. At least one manufacturer has utilized more than one inlet across the bottom of the chamber in order to create an even more turbulent air flow. However, it has been found that this solution, even when used with a baffle, does not adequately solve the particulate accumulation problem.
Particulate accumulation can cause other problems in a filter system other than noncleanliness. The particulates 19 within the enclosure can act as fuel for a fire, or “explosion.”Manufacturers have set limits for the amount of particulates 19 per unit volume that they consider a safe amount to be in the enclosure 14 at a given time. This limit is often referred to as the “lower explosion limit,” or “LEL,” and varies among different manufacturers. The limit is also referred to as the “lower flammability limit (“LFL”).” Calculation of the limit may or may not include the particulates 19 located on the outside of the filtration media 18. However, regardless of the limit set, the particulates 19 accumulated at the bottom of the enclosure 14 and at the corners 26 of the enclosure is included in the calculation.
To prevent any possible explosions in a filter enclosure from spreading to other parts of a plant, manufacturers often provide explosion vents (not shown) at the top of the enclosure 14. The explosion vents open when a certain pressure is built up within the enclosure 14. The vent provides an escape for igniting gases, and prevents an explosion from spreading to all parts of a plant. The explosion vent typically leads to a duct, which is vented to the outside of the plant. The duct usually leads from the explosion vent at the top of the enclosure up to and through the roof of the manufacturer's facility. The structure and installation of the explosion vent and its duct work can often be more elaborate and more expensive than the filter enclosure 14. Thus, manufacturers have searched for ways to avoid having to provide these explosion vents.
In some cases, it is necessary to add additional filter stages after the primary rotary drum filter stage to achieve the required air purity level. A multi-stage drum filter may be used in these cases. The type and quantity of additional filter stages can vary. Examples of filter stages include self-cleaning filter stages and passive filter stages. A self-cleaning filter is usually a filter that has an automatic method for cleaning itself without operator intervention. A passive filter typically refers to any filter that does not have self-cleaning capabilities, and usually refers to a pocket or bag filter. One advantage of passive filters is the ability to capture particulates within the pockets or bags, which keeps the particulates out of the airstream. As a result, the level of particulates are more easily maintained below the LEL, which helps minimize the risk of explosion in the passive filter stages. Passive filters are typically less expensive than self-cleaning filters because they do not require any type of automated self-cleaning machinery, but may not be feasible for use in processes where the air leaving the primary rotary drum filter stage has high concentrations of dust because the passive filters may become clogged quickly and require frequent maintenance and/or replacement.
As a result, for processes with relatively high dust concentrations remaining in the air following the rotary drum filter stage, a self-cleaning filter stage is commonly used after the primary rotary drum filter stage because the periods between maintenance events is often longer for a self-cleaning filter than for a passive filter in this type of environment.
The most common types of self-cleaning filters used for the filter stage after the primary rotary drum filter stage are cartridge final filters or disk filters. Cartridge final filters use a bank of cartridge filters that are periodically cleaned with a burst of compressed air. This compressed air cleaning is controlled by a control panel, which is activated based on the amount of pressure drop across the cartridges. When the pressure drop across the cartridges reaches a certain level, which has been set to indicate that the cartridges are dirty, the compressed air cleaning cycle is automatically initiated. However, when the compressed air cleaning cycle is initiated, the particulates are blown away from the cartridges and re-entrained into the air. As a result, it is difficult to control the level of particulates below the LEL with the cartridge final filters, which can increase the risk of explosion in the self-cleaning filter stages.
A disk filter is typically a secondary rotary drum filter stage that is positioned after the primary rotary drum filter stage. This secondary rotary drum filter stage is often shorter in length than the primary rotary drum filter stage, but the operational principles and features are the same as those for the primary rotary drum filter stage.
Therefore, in order to minimize costs and maximize efficiency of the multi-stage drum filter system, as well as avoiding the potential explosion risks that may be introduced by cartridge final filters, it is desirable to have a passive filter located in the second stage after the primary rotary drum filter stage, instead of a self-cleaning filter. As a result, it may be desirable to improve the efficiency of the primary drum filter stage so that the air leaving this stage has a lower concentration of dust that must be processed by the passive filters. It is also desirable to improve the performance of the primary rotary drum filter stage so that a pre-separator stage prior to the rotary drum filter stage is not required. Alternatively and/or additionally, it may be desirable to improve the holding capacity of the passive filters in the second stage so that the periods between maintenance and/or replacement are extended.